The present invention relates to a process for the preparation of 4,4'-dinitrodibenzyls from 4-nitrotoluenes by oxidative dimerization.
The preparation of 4,4'-dinitrodibenzyl from 4-nitrotoluene by oxidative dimerization with atmospheric oxygen in strongly alkaline solution has been disclosed and is described in, for example, Chem. Berichte, 26, 2232 (1893), J. Chem. Soc., 91, 2079 (1907), J. Amer. Chem. Soc., 75, 1325 (1953) and Org. Synth. Coll., Vol. IV, 367 (1963). The mechanism of the said reaction has also been thoroughly investigated (see, for example, J. Amer. Chem. Soc., 75, 3265 (1955), Abstr. of Papers, 135th Meeting Amer. Chem. Soc., April 5-14, 1960, pages 6-25; Monatsh. Chem., 34, 1011 (1913), J. Amer. Chem., Soc. 84, 4153 (1962) and J. Amer. Chem. Soc., 88, 5491 (1966)).
An improved process for the preparation of 4,4'-dinitrodibenzyl from 4-nitrotoluene by oxidative dimerization with oxygen in alkaline solution is described in J. Org. Chem. 26, 4162 (1961) and in U.S. Pat. No. 2,965,681. These processes are carried out in the presence of certain amines or ethers.
Furthermore, DE-OS (German Published Specification) No. 2,051,167 discloses a process for the preparation of 4,4'-dinitrodibenzyl in which the oxidative dimerization is carried out with air or oxygen in methanolic or ethanolic NaOH or KOH solution, and in which a special reactor is used.
Oxidative dimerization in strongly alkaline solution using air or oxygen is the only process which has hitherto been disclosed for the preparation of industrial amounts of dinitrodibenzyl. Other methods, such as the reaction of 4-nitrotoluene with 0.5 mol of diethyl oxalate and 1 mol of sodium methylate (Chem. Ber., 30, 1053 (1897), Ann. Chem., 436, 56 (1924)), the reaction of 4-nitrobenzyl chloride with alkaline stannate solution (German Pat. No. 39,381), the action of mercury oxide on .alpha.,.alpha.-bis[4-nitrobenzyl]hydrazine dissolved in chloroform (Chem. Ber., 33, 2710 (1910)) or the nitration of dibenzyl (J. Amer. Chem., 52, 5040 (1930), J. Chem. Soc. Jap. Ind. Chem. Sect., 61, 469 (1958)), have likewise been disclosed but have not hitherto aroused industrial interest.
The abovementioned processes for the preparation of 4,4'-dinitrodibenzyl by oxidation of 4-nitrotoluene with air or oxygen are, however, associated with considerable disadvantages. The amounts of bases used for the preparation of the reactive intermediate from 4-nitro-toluene are at least 4 to 5 mol, 10 mol of base in fact being preferred, per mol of 4-nitrotoluene. Furthermore, the reaction is normally carried out at very low concentrations of nitrotoluene (about 5% strength solutions) since otherwise the reaction times become too long. Nevertheless, even under these conditions reaction times of about 6 to 8 hours are necessary for the air oxidation. However, this means that the space/time yields are poor, which is at the expense of the economics of the processes mentioned. Furthermore, it is necessary, in order to avoid the formation of 4,4'-dinitrostilbene, to carry out the reaction at relatively low temperatures, that is to say with cooling. An additional disadvantage of the said processes is that the product results in very finely divided form and thus the filtration of the product is associated with great industrial difficulties. In addition, large amounts of intensely colored filtrates are produced, and their working up likewise demands considerable industrial effort.